Tetrafluoroethylene homopolymers, modified polytetrafluoroethylene and other fluoropolymers in the form of emulsions are used in various fields, such as in the field of coating compositions, utilizing those characteristics which the fluoropolymers have, such as nonstickiness, heat resistance, wear resistance, chemical resistance and low frictional characteristics.
Fluoropolymer emulsions can be obtained by subjecting at lease one fluoromonomer, such as tetrafluoroethylene [TFE], among others, to emulsion polymerization using a fluorine-containing surfactant comprising fluorine atoms as an emulsifier (cf. e.g. U.S. Pat. No. 2,559,752).
The aqueous fluoropolymer emulsions obtained by emulsion polymerization generally have a fluoropolymer concentration of about 15-35% by mass and, for industrial utilization thereof, the fluoropolymer emulsions are generally concentrated to a fluoropolymer concentration of about 40-75% by mass.
The aqueous fluoropolymer emulsions obtained by emulsion polymerization contain the fluorine-containing surfactant used as the emulsifier in carrying out the polymerization reaction. Fluorine-containing surfactants are generally expensive and, preferably, they should be removed.
Further, in cases where a fluoropolymer emulsion is used as a coating for instance, there arises the problem that some characteristics of the fluoropolymer are impaired when the amount of the fluorine-containing surfactant in the emulsion is large. Therefore, in recent years, it has been desired that the fluorine-containing surfactant be removed from the fluoropolymer emulsion. Thus, it is necessary to purify an aqueous fluoropolymer emulsion.
The aqueous fluoropolymer emulsion can be purified on the occasion of the concentration of the aqueous fluoropolymer emulsion. Known in the art as the method of concentrating aqueous fluoropolymer emulsions are concentration by membrane separation, electric concentration, concentration by evaporation, concentration by phase separation, and ion exchange concentration, among others. Among these concentration methods, concentration by evaporation and concentration by membrane separation are known to be capable of romoving fluorine-containing surfactants.
As for the concentration by evaporation, a method comprising concentrating aqueous fluoropolymer emulsions by evaporation under acidic conditions has been disclosed, and it is described that this method can remove the fluorine-containing surfactant (cf. e.g. WO 01/79332).
However, there is a problem that this method allows fluoropolymer particles to aggregate and precipitate and, as a result, the dispersion state of the fluoropolymer emulsion is damaged. Once the dispersion state has been damaged, it is generally difficult to restore the original dispersion state, so that it becomes impossible to obtain a purified emulsion.
As regards the concentration by membrane separation, it has been disclosed that the fluorine-containing surfactant in an aqueous fluoropolymer emulsion can be removed when a method of concentrating the aqueous fluoropolymer emulsion by membrane separation using a semipermeable ultrafiltration membrane is employed and pure water is added repeatedly after a certain extent of concentration (cf. e.g. Japanese Kokoku Publication H02-34971).
In this document, it is described that the content of the fluorine-containing surfactant can be reduced to about 0-0.1% by mass relative to the fluoropolymer. However, since the method of quantitating the fluorine-containing surfactant as described in this document is not very highly accurate, the fluorine-containing surfactant was not really removed to a satisfactory extent. Thus, the emulsion purified in the above manner has a problem, namely it cannot meet the recent demand for the removal of fluorine-containing surfactants.
In the case of concentration by membrane separation, for sufficiently removing the fluorine-containing surfactant, it is necessary to repeat the concentration step a number of times. Further, in the case of concentrating tetrafluoroethylene homopolymers, for instance, which are poor in mechanical stability, the tetrafluoroethylene homopolymers tend to form fibers on the semipermeable ultrafiltration membrane surface to block the membrane. Thus, a problem arises, namely the concentration efficiency decreases.
Concentration by phase separation is known to be a method capable of concentrating aqueous fluoropolymer emulsions while preventing the fluoropolymer from forming fibers without causing a decrease in dispersibility.
The concentration by phase separation generally comprises adding a nonionic surfactant to an aqueous fluoropolymer emulsion and heating the mixture to a temperature not lower than the cloud point of the nonionic surfactant to thereby cause separation into an aqueous phase and an oily phase (cf. e.g. U.S. Pat. No. 3,037,953). However, there is no description or suggestion about the removal of a fluorine-containing surfactant by concentration based on phase separation.
There is no description or suggestion about the removing a fluorine-containing surfactant by electric concentration and/or ion exchange concentration.